Monitoring of heart sounds

ABSTRACT

This document discusses, among other things, a system comprising an implantable medical device (IMD) including an implantable heart sound sensor circuit configured to produce an electrical heart sound signal representative of a heart sound of a subject and a processor circuit. The processor circuit is coupled to the heart sound sensor circuit and includes a detection circuit, a heart sound feature circuit and a trending circuit. The detection circuit configured to detect a physiologic perturbation and the heart sound feature circuit is configured to identify a heart sound feature in the electrical signal. The processor circuit is configured to trigger the heart sound feature circuit in relation to a detected physiologic perturbation. The trending circuit is configured to trend the heart sound feature in relation to a recurrence of the physiologic perturbation. The processor circuit is configured to declare a change in a physiologic condition of the patient according to the trending.

RELATED APPLICATION

This document is a continuation-in-part of commonly assigned, Haro etal., U.S. patent application Ser. No. 11/561,428, entitled“Respiration-Synchronized Heart Sound Trending,” which was filed Nov.20, 2006, now abandoned, and which is incorporated by reference herein.

BACKGROUND

Implantable medical devices (IMDs) are devices designed to be implantedinto a patient. Some examples of these devices include cardiac functionmanagement (CFM) devices such as implantable pacemakers, implantablecardioverter defibrillators (ICDs), cardiac resynchronization devices,and devices that include a combination of such capabilities. The devicesare typically used to treat patients using electrical or other therapyand to aid a physician or caregiver in patient diagnosis throughinternal monitoring of a patient's condition. The devices may includeone or more electrodes in communication with sense amplifiers to monitorelectrical heart activity within a patient, and often include one ormore sensors to monitor one or more other internal patient parameters.Other examples of implantable medical devices include implantablediagnostic devices, implantable insulin pumps, devices implanted toadminister drugs to a patient, or implantable devices with neuralstimulation capability.

Heart sounds are associated with mechanical vibrations from activity ofa patient's heart and the flow of blood through the heart. Heart soundsrecur with each cardiac cycle and are separated and classified accordingto the activity associated with the vibration. The first heart sound(S1) can be thought of as the vibrational sound made by the heart duringtensing of the mitral valve. The second heart sound (S2) can be thoughtof as marking the beginning of diastole. The third heart sound (S3) andfourth heart sound (S4) can be conceptualized as related to fillingpressures of the left ventricle during diastole. Heart sounds are usefulindications of proper or improper functioning of a patient's heart.

Overview

This document relates generally to systems, devices, and methods formonitoring one or more heart sounds. In example 1, a system includes animplantable medical device (IMD). The IMD includes an implantable heartsound sensor circuit configured to produce an electrical heart soundsignal representative of a heart sound of a subject and a processorcircuit. The processor circuit is coupled to the heart sound sensorcircuit and includes a detection circuit, a heart sound feature circuitand a trending circuit. The detection circuit configured to detect aphysiologic perturbation and the heart sound feature circuit isconfigured to identify a heart sound feature in the electrical signal.The processor circuit is configured to trigger the heart sound featurecircuit in relation to a detected physiologic perturbation. The trendingcircuit is configured to trend the heart sound feature in relation to arecurrence of the physiologic perturbation. The processor circuit isconfigured to declare a change in a physiologic condition of the patientaccording to the trending.

In example 2, the heart sound feature circuit of example 1 is optionallyconfigured to identify a plurality of candidate heart sound features.The processor circuit optionally includes a ranking circuit configuredto rank the candidate heart sound features, and select a heart soundfeature for trending according to the ranking.

In example 3, the ranking circuit of example 2 is optionally configuredto rank the candidate features according to a physiologic condition ofthe subject.

In example 4, the IMD of examples 1-4 optionally includes a secondimplantable sensor circuit in electrical communication with theprocessor circuit. The implantable sensor circuit is configured toproduce a second electrical sensor signal related to one or morephysiologic cardiovascular events in the subject. The detection circuitof examples 1-4 is optionally configured to detect the physiologicperturbation from the second electrical sensor signal.

In example 5, the second implantable sensor circuit of example 4optionally includes a cardiac signal sensing circuit configured toprovide an electrical representative of an intrinsic cardiac signal ofthe subject. The detection circuit is optionally configured to detect apremature ventricular contraction (PVC) to trigger the heart soundfeature circuit.

In example 6, the second implantable sensor circuit of example 4optionally includes an activity sensor to provide a signalrepresentative of a physical activity level of the subject. Thedetection circuit is optionally configured to detect that a physicalactivity level of the subject exceeds a threshold activity level fromthe signal to trigger the heart sound feature circuit.

In example 7, the second implantable sensor circuit of example 4optionally includes a posture sensor, and the detection circuit isoptionally configured to detect a change in posture of the subject totrigger the heart sound feature circuit.

In example 8, the second implantable sensor circuit of example 4optionally includes a cardiac signal sensing circuit configured toprovide an electrical representative of an intrinsic cardiac signal ofthe subject. The detection circuit is optionally configured to detectthat a heart rate of the subject exceeds a threshold heart rate totrigger the heart sound feature circuit.

In example 9, the second implantable sensor circuit of example 4optionally includes a respiration sensor to provide a signalrepresentative of respiration of the subject. The detection circuit isoptionally configured to detect a phase of a respiration cycle of thesubject from the signal to trigger the heart sound feature circuit.

In example 10, the processor circuit of examples 1-9 optionally includesa timer circuit, and the processor circuit is optionally configured totrigger the heart sound feature circuit in relation to a time thesubject receives a drug therapy.

In example 11, the processor circuit of examples 1-10 optionallyincludes a timer circuit, and the processor circuit is optionallyconfigured to trigger the heart sound feature circuit in relation to aphase of a circadian cycle of the subject.

In example 12, the IMD of examples 1-11 optionally includes a therapycircuit in electrical communication with the processor circuit. Thetherapy circuit is configured to provide a therapy to the subject, andthe processor circuit is optionally configured to initiate a therapy tocause the physiologic perturbation and to trigger the heart soundfeature circuit in a time relation to the therapy.

In example 13, the processor circuit of example 12 is optionallyconfigured to initiate a paced cardiac contraction as the physiologicperturbation and to trigger the heart sound feature circuit in a timerelation to the paced cardiac contraction.

In example 14, the processor circuit of examples 12 and 13 is optionallyconfigured to initiate a premature ventricular contraction (PVC) as thephysiologic perturbation and to trigger the heart sound feature circuitin a time relation to the PVC.

In example 15, a method includes sensing an electrical heart soundsignal representative of a heart sound of a subject in relation to aphysiologic perturbation using an IMD, identifying a feature of theheart sound signal, trending the feature of the heart sound signal inrelation to a recurrence of the physiologic perturbation, andidentifying a change in a physiologic condition of the subject from thetrending.

In example 16, identifying the feature of example 15 optionally includesidentifying a plurality of candidate features of the heart sound signal,ranking the candidate features, and selecting a feature for trendingaccording to the ranking.

In example 17, ranking the candidate features of example 16 optionallyincludes ranking the candidate features according to a physiologiccondition of the subject.

In example 18, identifying the feature of the heart sound signal ofexamples 15-17 optionally includes triggering identification of thefeature of the heart sound in relation to a detected physiologicperturbation.

In example 19, identifying the feature of the heart sound signal ofexamples 15-18 optionally includes triggering identification of thefeature of the heart sound in relation to an induced physiologicperturbation.

In example 20, the physiologic perturbation of examples 15-19 optionallyincludes a premature ventricular contraction (PVC), and identifying thefeature of the heart sound signal optionally includes sensing a heartsound signal includes sensing a heart sound signal in relation to thePVC.

In example 21, the physiologic perturbation of examples 15-20 optionallyincludes a change in posture of the subject, and sensing the heart soundsignal optionally includes sensing a heart sound signal in relation tothe change in posture.

In example 22, the physiologic perturbation of examples 15-21 optionallyincludes a heart rate of the subject exceeding a threshold heart rate,and sensing the heart sound signal optionally includes sensing a heartsound signal in relation to the heart rate of the subject exceeding thethreshold heart rate.

In example 23, the physiologic perturbation of examples 15-22 optionallyincludes the subject receiving drug therapy, and sensing the heart soundsignal optionally includes sensing a heart sound signal in relation to atime the subject receives the drug therapy.

In example 24, the physiologic perturbation of examples 15-23 optionallyincludes a paced cardiac contraction, and sensing the heart sound signaloptionally includes sensing the heart sound signal in relation to thepaced cardiac contraction.

In example 25, the physiologic perturbation of examples 15-24 optionallyincludes a phase of a respiration cycle, and sensing the heart soundsignal optionally includes sensing the heart sound signal in relation tothe phase of a respiration cycle.

In example 26, the physiologic perturbation of examples 15-25 optionallycorresponds to a circadian pattern, and the sensing the heart soundsignal optionally includes sensing the heart sound signal in relation tothe circadian pattern.

In example 27, the physiologic perturbation of examples 15-26 optionallyincludes activity of the subject exceeding a threshold activity level,and sensing the heart sound signal optionally includes sensing the heartsound signal in relation to the activity level of the subject exceedingthe threshold activity level.

In example 28, a system includes means for implantably sensing anelectrical heart sound signal representative of a heart sound of asubject, means for identifying a feature of the heart sound signal inrelation to a detected physiologic perturbation, means for trending thefeature of the heart sound signal in relation to a recurrence of thephysiologic perturbation, and means for identifying a change in aphysiologic condition of the subject from the trending.

In example 29, a system example includes an IMD and an external device.The IMD includes an implantable heart sound sensor circuit configured toproduce an electrical heart sound signal representative of a heart soundof a subject and a first processor circuit coupled to the heart soundsensor circuit. The processor circuit includes a detection circuitconfigured to detect a physiologic perturbation; and a heart soundfeature circuit configured to identify a heart sound feature in theelectrical signal. The processor circuit is configured to trigger theheart sound feature circuit in relation to a detected physiologicperturbation. The external device includes a second processor circuitwhich includes a trending circuit configured to trend the heart soundfeature in relation to a recurrence of the physiologic perturbation. Thesecond processor circuit is configured to declare a change in aphysiologic condition of the patient according to the trending.

In example 30, the heart sound feature circuit of example 29 isoptionally configured to identify a plurality of candidate heart soundfeatures. The first processor circuit is optionally configured tocommunicate information about the heart sound signal features to theexternal device. The second processor circuit optionally includes aranking circuit configured to rank the candidate heart sound features,and select a heart sound feature for trending according to the ranking.

In example 31, the ranking circuit of example 30 is optionallyconfigured to rank the candidate features according to a physiologiccondition of the subject.

This overview is intended to provide an overview of subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the presentpatent application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 is an illustration of a graph of S1 and S2 heart sounds.

FIG. 2 shows a block diagram of portions of an example of a system formonitoring one or more heart sounds.

FIG. 3 illustrates an example of an IMD coupled to a heart by one ormore leads.

FIG. 4 shows a graph illustrating S2-S1 time intervals for inspirationand expiration.

FIG. 5 shows a graph illustrating S1 and S2 heart sounds without pacingthe left ventricle (LV), and a graph illustrating heart sounds withpacing the LV.

FIG. 6 shows a flow diagram of an example of a method for monitoringheart sounds.

FIG. 7 shows a heart sound graph of S1-S4 heart sounds recordedsynchronously with a LV pressure graph and an aortic pressure graph.

FIG. 8 shows graphical representations of the decomposition of anelectrical signal obtained from a heart sound sensor.

FIG. 9 shows a block diagram of portions of another example of a systemfor monitoring one or more heart sounds.

FIG. 10 shows a block diagram of portions of a further example of asystem for monitoring one or more heart sounds.

DETAILED DESCRIPTION

Because heart sounds are a mechanical measure of a patient's hemodynamicsystem, monitoring of one or more heart sounds can aid a caregiver indetecting overall progression of heart disease.

FIG. 1 is an illustration of a graph of the S1 115 and S2 120 heartsounds. The mechanical events indicated by heart sounds change whenthere is a physiologic or patho-physiologic change in the cardiovascularsystem. These changes can be viewed as physiologic perturbations. Forexample, when a patient exercises, their heart rate and cardiac strokevolume increases. The increase in heart rate decreases the cardiac cyclelength and shifts the S1 and S2 heart sounds in time. The increase instroke volume changes the amplitudes of the S1 and S2 heart sounds.Other examples of such physiologic perturbations include, withoutlimitation, a change in patient posture, one or more prematureventricular contractions (PVCs), drug therapy received by the patient, apaced cardiac contraction, and the inhaling or exhaling of the patient.A specific perturbation causes an expected change in the heart sounds.Monitoring heart sounds when the specific perturbation occurs mayprovide additional information about the physiologic condition of thepatient or subject.

FIG. 2 shows a block diagram of portions of an example of a system 200for monitoring one or more heart sounds. The system 200 includes animplantable medical device (IMD) 205 that in turn includes a heart soundsensor circuit 210. Heart sound sensors generally include implantableacoustic sensors that convert the detected sounds of the heart into anelectrical signal representative of the heart sounds. In some examples,the heart sound sensor circuit 210 includes an accelerometer locatedwithin a hermetically sealed canister or “can” of the IMD 205. Inanother sensor example, a microphone is located within the IMD can. Inanother example, the heart sound sensor circuit 210 includes a straingauge.

The system 200 also includes a processor circuit 215 coupled to theheart sound sensor circuit 210. In some examples, the processor circuit215 is coupled to the heart sound sensor circuit 210 through a heartsound sensor interface circuit 220. The heart sound sensor interfacecircuit 220 provides signal conditioning such as signal filtering and/oramplification for example.

The processor circuit 215 includes a detection circuit 225, a heartsound feature circuit 230, and a trending circuit 235. The processorcircuit 215 can be implemented using hardware circuits, firmware,software or any combination of hardware, firmware and software.Examples, include a microprocessor, a logical state machine, and aprocessor such as a microprocessor, application specific integratedcircuit (ASIC), or other type of processor. The processor circuit 215 isconfigured to perform or execute a function or functions. Such functionscorrespond to circuits or other type of modules, which are software,hardware, firmware or any combination thereof. Multiple functions may beperformed in one or more of the circuits.

The detection circuit 225 detects a physiologic perturbation. When aperturbation is detected, the processor circuit 215 triggers the heartsound feature circuit 230 in relation to the perturbation to identify aheart sound feature in the electrical signal representative of the heartsound.

In some examples, the detection circuit 225 detects the physiologicperturbation using the electrical signal from the heart sound sensorcircuit 210. For example, if the physiologic perturbation is patientexercise, the detection circuit 225 may detect exercise from a shift inS1 and S2 heart sounds. If the heart sound sensor circuit 210 includesan accelerometer, the accelerometer may double as an activity sensor andthe detection circuit 225 may detect patient activity such as exerciseusing the accelerometer. Descriptions of systems to monitor heart anddetect patient activity may be found in Maile et al., U.S. PatentApplication Publication No. 20050102001, entitled “Dual-Use Sensor forRate Responsive Pacing and Heart Sound Monitoring, filed Nov. 6, 2003,now issued as U.S. Pat. No. 7,248,923, which is incorporated herein byreference.

According to some examples, the detection circuit 225 may detect thephysiologic perturbation using a second implantable sensor circuit 240.The second implantable sensor circuit 240 produces a second electricalsensor signal related to one or more physiologic cardiovascular eventsin the subject. For example, if the physiologic perturbation is patientexercise, the second implantable sensor circuit 240 includes an activitysensor to provide a signal representative of an activity level of thesubject, such as an accelerometer for example. The detection circuit 225is configured to detect that a physical activity level of the subjectexceeds a threshold activity level from the signal.

As described above, when a patient exercises, their heart rate andcardiac stroke volume increases. The increase in heart rate decreasesthe cardiac cycle length and shifts the S1 and S2 heart sounds in time.The increase in stroke volume due to the exercise changes the amplitudesof the S1 and S2 heart sounds. Therefore, a shift in the S1 and S2 heartsounds and an increase in amplitudes of one or both heart sound arefeatures that correlate to stroke volume and can be used to monitor aphysiologic change in stroke volume.

In some examples, the physiologic perturbation is a change in posture ofthe patient. The second implantable sensor circuit 240 includes aposture sensor and the detection circuit 225 is configured to detect achange in posture of the subject. Changes in contractility of thesubject's heart are reflected in heart sounds. A change in posturechanges the preload of the heart. A change in contractility may bereflected in a change in timing intervals of the heart sounds and/or inthe amplitude of the S1 heart sound. For example, in a weak leftventricle the isovolumic contraction time is prolonged; resulting in awide S1 complex or long S1 duration. Also, the occurrence of the S1complex may be delayed. Therefore, S1 width, S1 amplitude, and a timeshift in S1 are features that correlate to contractility and can be usedto monitor a physiologic change in contractility.

In some examples, the physiologic perturbation is a change in phase of acircadian cycle or pattern of the subject, (e.g., waking or sleeping).The processor circuit 215 may include a timer circuit 245. The processorcircuit 215 triggers the heart sound feature circuit to identify a heartsound feature in relation to a phase of a circadian pattern of thesubject.

In some examples, the physiologic perturbation is a drug therapyreceived by the subject. If the heart sounds change (e.g., amplitude) inresponse to a drug therapy (e.g., a diuretic intake), this may indicatethat the renal function is not resistant to the drug therapy. Therefore,a change in amplitude of heart sounds may be a feature that correlatesto a drug therapy's impact on renal function. In certain examples, theprocessor circuit uses the timer circuit 245 to trigger the heart soundfeature circuit in relation to a time the subject receives the drugtherapy. In some examples, the IMD 205 includes a drug reservoir andadministers a drug therapy to the subject such as by a positivedisplacement pump mechanism for example. The processor circuit 215triggers the heart sound feature circuit in a time relation to adelivery of drug therapy to the subject.

FIG. 3 illustrates an example of an IMD 310 coupled to heart 305, suchas by one or more leads 308A-B. Heart 305 includes a right atrium 300A,a left atrium 300B, a right ventricle 305A, a left ventricle 305B, and acoronary vein 320 extending from right atrium 300A. In this embodiment,atrial lead 308A includes electrodes (electrical contacts, such as ringelectrode 325 and tip electrode 330) disposed in, around, or near aright atrium 300A of heart 305 for sensing signals, or delivering pacingtherapy, or both, to the right atrium 300A. Lead 308A optionally alsoincludes additional electrodes, such as for delivering atrialcardioversion, atrial defibrillation, ventricular cardioversion,ventricular defibrillation, or combinations thereof to heart 305. Lead308A optionally further includes additional electrodes for deliveringpacing or resynchronization therapy to the heart 305.

Ventricular lead 308B includes one or more electrodes, such as tipelectrode 335 and ring electrode 340, for sensing signals, fordelivering pacing therapy, or for both sensing signals and deliveringpacing therapy. Lead 308B optionally also includes additionalelectrodes, such as for delivering atrial cardioversion, atrialdefibrillation, ventricular cardioversion, ventricular defibrillation,or combinations thereof to heart 305. Such electrodes typically havelarger surface areas than pacing electrodes in order to handle thelarger energies involved in defibrillation. Lead 308B optionally furtherincludes additional electrodes for delivering pacing orresynchronization therapy to the heart 305.

Other forms of electrodes include meshes and patches which may beapplied to portions of heart 305 or which may be implanted in otherareas of the body to help “steer” electrical currents produced by IMD310. In one embodiment, one of atrial lead 308A or ventricular lead 308Bis omitted, i.e., a “single chamber” device is provided, rather than thedual chamber device illustrated in FIG. 3. In another embodiment,additional leads are provided for coupling the IMD 310 to other heartchambers and/or other locations in the same heart chamber as one or moreof leads 308A-B. The present methods and systems will work in a varietyof configurations and with a variety of electrical contacts or“electrodes,” including a leadless system that uses electrodes remotefrom, rather than touching, the heart 305.

Returning to FIG. 2, in some examples, the second implantable sensorcircuit 240 includes a cardiac signal sensing circuit. The cardiacsignal sensing circuit includes one or more sense amplifiers inelectrical communication with electrodes to provide an electricalrepresentative of an intrinsic cardiac signal of the subject.

Baroreflex sensitivity (BRS) is a measure of the gain in the resultingrecovery in blood pressure and is typically measured using units ofmilliseconds per millimeters of mercury (ms/mmHg). BRS has been shown tobe a good prognostic indicator. For example, Mean arterial pressure(MAP) recovery is used to assess a patient's hemodynamic tolerance to atachyarrhythmia. BRS correlates well to MAP recovery during ventriculartachyarrhythmia and for this reason BRS is a good measure of hemodynamicstability during tachyarrhythmia. An indication of a subject's BRS canbe established by measuring blood pressure and monitoring heart rate.

In certain examples, the physiologic perturbation includes a sensedpremature ventricular contraction (PVC) and the detection circuit 225 isconfigured to detect a PVC. A PVC refers to two ventricular contractionsoccur (V-V interval), without an intervening a trial contraction. Anindication of BRS indicator is a measure of the gain in the resultingrecovery in blood pressure after the PVC. The gain can be viewed as theslope of a graph of V-V intervals versus change in blood pressure. Ahigher slope reflects higher BRS and a lower slope reflects lower BRS.Descriptions of systems and methods that monitor the baroreflexsensitivity of a subject are found in Ettori et al., U.S. patentapplication Ser. No. 11/457,644, “Baroreflex Sensitivity Monitoring andTrending for Tachyarrhythmia Detection,” filed Jul. 14, 2006, publishedas U.S. Patent Application Publication No. 20080015651, which isincorporated herein by reference.

A change in blood pressure may be reflected in a change in heart soundssuch as the amplitude of the heart sounds and/or the power spectrum ofthe heart sounds for example. Therefore, heart sound amplitude and heartsound power spectrum are features that correlate to blood pressure andcan be used as a surrogate to monitor a physiologic change in BRS.

In certain examples, the physiologic perturbation includes a heart ratethat exceeds a threshold heart rate, and the detection circuit 225 isconfigured to detect when a heart rate of the subject exceeds athreshold heart rate value. A change in one or more heart soundintervals are features that are useful to monitor heart ratevariability.

According to some examples, the physiologic perturbation includesmodulation of the subject's hemodynamics due to respiration. In certainexamples, the second implantable sensor circuit 240 includes arespiration sensor to provide a signal representative of respiration ofthe subject, and the detection circuit 225 is configured to detect aphase of a respiration cycle of the subject from the signal. An exampleof an implantable respiration sensor is a transthoracic impedance sensorto measure minute respiration volume. An approach to measuringtransthoracic impedance is described in Hartley et al., U.S. Pat. No.6,076,015, “Rate Adaptive Cardiac Rhythm Management Device UsingTransthoracic Impedance,” filed Feb. 27, 1998, which is incorporatedherein by reference.

Modulation of hemodynamics due to respiration is another example of aphysiologic perturbation that changes heart sounds. Duringend-inspiration, the cardiac stroke volume may increase which in turnincreases heart sound amplitude. Also, an increase in cardiac strokevolume shifts the timing of the opening and closing of the cardiacvalves and hence shifts the timing of the heart sounds.

FIG. 4 shows a graph 400 illustrating S2-S1 time intervals forinspiration and expiration. The graph 400 shows that the S2-S1 timeinterval is longer after inspiration than expiration. This difference istime intervals is due to a change in stroke volume and is also reflectedin a difference in left ventricle (LV) pressure. The graph shows that LVpressure is higher for inspiration, and thus the amplitude of the S2 andS1 heart sounds will be greater for inspiration. Therefore, the S2-S1interval and the heart sound amplitude are features that correlate tostroke volume and can be used to monitor a physiologic change in strokevolume. Descriptions of systems and methods for monitoring heart soundsin relation to respiration are found in the above mentioned Haro et al.,“Respiration-Synchronized Heart Sound Trending,” which is incorporatedherein by reference.

As described above, heart sound features may correlate to a drugtherapy's impact on renal function. A drug therapy's impact on renalfunction may also be reflected in a change in thoracic impedance. Thus,measuring transthoracic impedance in conjunction with monitoring heartsounds is useful to monitor a drug therapy's impact on renal function.

When a perturbation is detected using the detection circuit 225 of FIG.2, the processor circuit 215 triggers the heart sound feature circuit230 in relation to the perturbation to identify a heart sound feature inthe electrical signal representative of the heart sound. Examples ofheart sound features include, without limitation, an interval betweenheart sounds (e.g., an interval between the S1 and the S2 heart soundsin FIG. 1), a duration of a heart sound (e.g., the duration or width ofthe S1 heart sound), an amplitude of a peak of a heart sound (e.g., azero-to-peak amplitude or a peak-to-peak amplitude of a heart sound) ora power spectrum of a heart sound. Descriptions of systems and methodsfor additional heart sound measurements are found in Patangay et al.,U.S. patent application Ser. No. 11/625,003, “Ischemia Detection UsingHeart Sound Timing,” filed Jan. 19, 2007, published as U.S. PatentApplication Publication No. 20080177191, now issued as U.S. Pat. No.7,736,319, which is incorporated by reference.

The trending circuit 235 trends the identified heart sound features inrelation to a recurrence of the physiologic perturbation. The goal is tochoose a heart sound feature providing the desired information about thepatient, identify the feature in relation to the physiologicperturbation using the heart sound feature circuit 230, and trend thefeature over time to determine whether there is a change in aphysiologic condition of the patient according to the trending. Thetrending determines a measurable margin of change in an aspect of thepathological condition to uncover a subclinical pathologic change.Examples of the trending include, without limitation, trending todetermine a change in decompensation (heart failure), a change insympathetic activity, a change in patient response to a drug therapy,and a change in cardiac contractility.

In some examples, the IMD 205 includes a communication circuit tocommunicate information wirelessly to an external device, such as bymutual induction, radio frequency (RF), or another telemetry method. TheIMD 205 communicates an indication of the change in the physiologiccondition to an external device. The indication is then available to auser, such as a clinician or caregiver, or is available to a thirddevice in communication with the external device.

In some examples, the IMD does not necessarily include the trendingcircuit 235, and the trending is done by the external device, such as anIMD programmer or a server that is part of a patient management system.The IMD 205 includes a communication circuit to communicate informationabout one or more heart sound features to the external device. Theexternal device trends a feature over time to determine whether there isa change in a physiologic condition of the patient according to thetrending.

The physiologic perturbation used to trigger the feature identificationof the heart sound signal may be sensed and occur naturally, or theperturbation may be induced. The perturbation may be induced by causingsome action by the subject (e.g., forced breathing by the patient tocause a PVC) or the perturbation may be induced by the IMD 205. In someexamples, the IMD 205 includes a therapy circuit 250. The therapycircuit 250 provides a therapy (e.g., pacing therapy) to the subject.The processor circuit 215 initiates a therapy to induce the physiologicperturbation and to trigger the heart sound feature circuit 230 in atime relation to the therapy.

A pacing pulse provided by the therapy circuit 250 can be viewed as aninduced PVC. For example, the processor circuit 215 may initiate twoventricular contractions or a ventricular contraction after an intrinsicventricular contraction before an intervening atrial contraction occurs.The processor circuit 215 initiates the PVC and triggers the heart soundfeature circuit 230 in a time relation to the PVC. Thus, heart soundfeatures can be monitored in relation to a PVC without having to waitfor a sensed occurrence of a PVC.

Because pacing therapy may cause a change in synchrony of a cardiaccontraction, pacing therapy is also a physiologic perturbation. If thechambers (e.g., the ventricles) of the heart are dyssynchronous, pacingsynchronizes the contraction by activating delayed myocardial regionswith those regions that activate earlier. Subjects with heart failure(HF) may have a dyssynchrony of heart chambers. Pacing the chambers torestore coordination of the contractions increases the stroke volume ofthe heart and therefore changes the heart sounds.

FIG. 5 shows a graph 505 illustrating S1 and S2 heart sounds withoutpacing the left ventricle (LV) to synchronize contractions with theright ventricle (RV), and a graph 510 illustrating heart sounds withpacing to synchronize the LV with the RV. It can be seen in the graphsthat pacing the left ventricle synchronous with the right ventricleincreases the amplitude of the S1 and S2 heart sound. In the IMD of FIG.2, the processor circuit 215 initiates a paced cardiac contraction asthe physical perturbation and triggers the heart sound feature circuit230 in a time relation to the paced cardiac contraction.

FIG. 6 shows a flow diagram of an example of a method 600 for monitoringheart sounds. At 605, an electrical heart sound signal representative ofa heart sound of a subject is implantably sensed. At 610, a feature ofthe heart sound signal is identified in relation to a detectedphysiologic perturbation. At 615, the feature is trended in relation toa recurrence of the physiologic perturbation. At 620, a change in aphysiologic condition of the subject is identified from the trending.

Many heart sound features have been described as being of interest todifferent physiologic pathologies. A non-exhaustive list of thesefeatures have included durations of heart sounds, intervals betweenheart sounds, amplitudes of heart sounds and power spectrums of heartsounds. Thus, heart sounds have multiple components or features and thefeatures may be represented in the time domain or the frequency domain.

FIG. 7 shows a heart sound graph 705 of the S1-S4 heart sounds recordedsynchronously with a LV pressure graph 710 and an aortic pressure graph715. It can be seen in the graphs that the different components of theS1 waveform have different frequency localization properties. Forexample, the early portion of S1, associated with the mitral valve has ahigher frequency than the later portion of the S1 heart sound associatedwith blood ejecting after opening of the aortic valve. The mitralcomponent is typically localized around 30-60 hertz (Hz) whereas theaortic component is typically localized around 15-25 Hz.

While there is much information regarding the localization andamplitudes of various heart sounds components contained in thetime-frequency domain of heart sounds, this information is notconsistent across different patho-physiologic states. For example, inpatients with reduced contractility with mitral regurgitation, themitral component of the S1 heart sound is diminished due to leakyvalves. Each patient or subject may have a unique pathology that givingthe heart sounds of the patient a unique time-frequency footprint withpatho-physiologic information related to the patient's disease state.This time-frequency footprint contains information that, when extracted,can be used for clinical applications.

Wavelet filtering can be used to extract information from signalsgenerated by heart sound sensors to detect valvular regurgitation (VR).Wavelet analysis decomposes an electrical signal with both frequency andtime. Therefore, the signal energy information includes variations ofthe amplitude of the electrical signal with both frequency and time.

In wavelet analysis, a scalable modulated window is typically shiftedalong the time domain electrical signal and for every position thefrequency spectrum is calculated. This process is typically repeatedmany times with a slightly shorter (or longer) window for every newcycle of the electrical signal. By using a variable width window,wavelet analysis effectively zooms in on the signal when analyzinghigher frequencies, thus providing higher resolution when necessary. Theresult is a collection of time-frequency representations of the signalhaving different resolutions.

The time-frequency representations of the signal can then be decomposedinto component signals. Many different wavelet functions can be used todecompose the input electrical signal into component parts. In someexamples, Daubechies wavelets are used. The ability of a waveletfunction to decompose a signal into its component parts depends on howclosely the wavelet used approximates the electrical signal.

FIG. 8 shows graphical representations 800 of the decomposition of theelectrical signal obtained from the heart sound sensor. An electricalsignal 810 is shown in the top graph. In this example, the decompositionis performed by running the electrical signal 810 through a bank ofbandpass filters corresponding to the Daubechies wavelets to obtain thesix individual decomposed element signals a5, d5, d4, d3, d2, and d1.

In some examples, the heart sound feature circuit 230 includes a waveletfilter circuit to decompose an electrical heart sound signal intocomponent signals. Descriptions of systems and methods for monitoringheart sounds using wavelet filtering are found in Patangay et al., U.S.Patent Application Publication No. 20070123943, “Systems and Methods forValvular Regurgitation Detection,” filed Nov. 28, 2005, now issued asU.S. Pat. No. 8,108,034, which is incorporated herein by reference.

In some examples, once a feature is identified, the heart sound featurecircuit 230 preprocesses subsequent heart sound signals (e.g., fromsubsequent cardiac cycles) by decomposing the heart sound signals intocomponent signals. In certain examples, the heart sound feature circuitincludes a threshold detector to detect one or more heart sound featuresof interest.

In some examples, the heart sound feature circuit 230 of FIG. 2identifies heart sound signal features using a comparison to a heartsound template. The IMD 205 includes a memory to store one or more heartsound templates, which include previously attained heart soundinformation from the subject. The heart sound template includes at leastone heart sound feature, such as a morphological feature. The heartsound feature circuit 230 uses a cross-correlation between a heart soundsignal and at least a portion of the heart sound template to identifyfeatures in the heart sound signal. Descriptions of system and methodsto identify heart sound features using a heart sound template are foundin Patangay et al., U.S. patent application Ser. No. 11/736,055, “HeartSound Tracking System and Method,” filed Apr. 17, 2007, published asU.S. Patent Application Publication No. 20080262368, now issued as U.S.Pat. No. 7,853,327, which is incorporated by reference.

There may be difficulty in identifying features in heart sound signalsfor trending over subsequent cardiac cycles because variation infeatures may cause feature misdetections. A solution is to useinformation from previous and/or the current heart sound beat to reducethe misdetections. In some examples, the heart sound feature circuit 230calculates a feature metric. A heart sound feature is tracked byoptimizing the feature metric to find the most consistent heart soundfeature or set of features. Descriptions of systems and methods to trackheart sound features using optimization of a feature metric are found inthe above mention Patangay et al., “Heart Sound Tracking System andMethod.”

Different pathologies of a patient may emphasize different individualcomponents or features of heart sounds. As described above, a pathologymay include a change to multiple heart sound features. Althoughdetecting an occurrence of a heart sound, such as S1 , may be relativelystraight-forward, determining which feature of the heart sound is themost sensitive to changes in systolic performance results in analyzingthe heart sound complex over time. Because it is desired to find anoptimum feature for trending, the physiologic perturbation is used tocreate a change in the features of the heart sound. When multiple heartsound signal features or components are identified, the identifiedcandidate features can be ranked to determine which feature allows forcorrect and consistent physiologic variable measurements.

FIG. 9 shows a block diagram of portions of another example of a system900 for monitoring one or more heart sounds. The system 900 includes anIMD 905 that in turn includes an implantable heart sound sensor circuit910 and processor circuit 915. The processor circuit includes adetection circuit 925 to detect a physiologic perturbation and a heartsound feature circuit 930 to identify one or more heart sound featuresin the electrical heart sound signal. The processor circuit 915 isconfigured to trigger the heart sound feature circuit 930 in relation toa detected physiologic perturbation.

The processor circuit 915 also includes a ranking circuit 955 to rankidentified candidate heart sound features and select a heart soundfeature for trending according to the ranking. The ranking circuit 955may rank the candidate features by giving a greater weight to candidatefeatures of interest to the physiologic condition of the subject. Forexample, the ranking circuit 955 may give greater weight to a durationor width of the S1 heart sound when the physiologic condition of thesubject relates to contractility and the IMD 905 is searching for afeature to trend contractility, than when the physiologic condition ofthe subject relates to drug therapy and the IMD 905 is searching for afeature to trend efficacy of the drug therapy.

According to some examples, the ranking circuit 955 uses a lineardiscriminant analysis classifier to rank the features. In certainexamples, the linear discriminant analysis classifier uses Fischer'sseparability criterion to rank variables to train the classifier. In aregression problem where a set of inputs is used to predict an outputvariable, a correlation coefficient can be used to estimate thevariability in the output due to a given input. A linear or non-linearfit can be used to calculate the correlation coefficient. Weighting canalso be added to certain candidate features during the training of theclassifier.

In some examples, the ranking circuit 955 uses entropy based(information theoretic) criteria to rank the candidate features. Theestimated densities of discrete variables are used to calculate themutual information between the input variables and the output to rankthe candidate features. In some examples, the ranking circuit 955 ranksthe candidate features using Chi-Square analysis. In some examples, theranking circuit 955 ranks the candidate features using an analysis ofvariance (ANOVA) type statistical tests to rank the candidate features.

Once a heart signal feature or set of features to trend has beenidentified from the candidate features, the trending circuit 935 trendsthe heart sound feature in relation to a recurrence of the physiologicperturbation. The processor circuit 915 declares a change in aphysiologic condition of the patient according to the trending.

In some examples, the trending and the ranking are done by a seconddevice. FIG. 10 shows a block diagram of portions of another example ofa system 1000 for monitoring one or more heart sounds. The system 1000includes an IMD 1005 and an external device 1070. The IMD 1005 includesan implantable heart sound sensor circuit 1010 and a first processorcircuit 1015. The processor circuit 1015 includes a detection circuit1025 to detect a physiologic perturbation and a heart sound featurecircuit 1030 to identify one or more heart sound features in theelectrical heart sound signal. The processor circuit 1015 triggers theheart sound feature circuit 1030 in relation to a detected physiologicperturbation. The processor circuit 1015 communicates information aboutone or more heart sound signal features to the external device using thecommunication circuit 1060.

The external device 1070 includes a second processor circuit 1075 andcommunication circuit 1065 to communicate information with the IMD.Examples of the external device include an IMD programmer or a server.The IMD 1005 may communicate to the external device 1070 using a thirddevice, such as a repeater for example. The processor circuit 1075includes a trending circuit 1035 to trend the heart sound feature inrelation to a recurrence of the physiologic perturbation. The processorcircuit 1075 declares a change in a physiologic condition of the patientaccording to the trending. The processor circuit 1075 may provide anindication of the change to a user or a caregiver, such as by a displayon the external device 1070 for example. The processor circuit 1075 mayprovide the indication to another process, such as a third device incommunication with the external device via a communication network suchas a computer network (e.g., the internet) or a cell phone network.

In some examples, the heart sound feature circuit 1030 of the IMD 1005identifies a plurality of candidate heart sound features and theprocessor circuit 1015 communicates information about the heart soundsignal features to the external device using the communication circuit1060. The processor 1075 includes a ranking circuit 1055 to rank thecandidate heart sound features and select a heart sound feature fortrending according to the ranking.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” All publications, patents, and patent documentsreferred to in this document are incorporated by reference herein intheir entirety, as though individually incorporated by reference. In theevent of inconsistent usages between this document and those documentsso incorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B.” “B but not A,” and “A and B,” unlessotherwise indicated. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the respectiveterms “comprising” and “wherein.” Also, in the following claims, theterms “including” and “comprising” are open-ended, that is, a system,device, article, or process that includes elements in addition to thoselisted after such a term in a claim are still deemed to fall within thescope of that claim. Moreover, in the following claims, the terms“first,” “second,” and “third,” etc. are used merely as labels, and arenot intended to impose numerical requirements on their objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, the code may be tangibly stored on one ormore volatile or non-volatile computer-readable media during executionor at other times. These computer-readable media may include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAM's), read onlymemories (ROM's), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A system comprising: an implantable heart sound sensor circuitconfigured to produce an electrical heart sound signal representative ofa heart sound of a subject; a second implantable sensor circuitconfigured to produce a second electrical sensor signal related to oneor more physiologic cardiovascular events in the subject; a processorcircuit coupled to the heart sound sensor circuit and the second sensorcircuit, wherein the processor circuit includes: a heart sound featurecircuit configured to identify a heart sound feature in the electricalheart sound signal in an absence of a physiologic perturbation; adetection circuit configured to detect the physiologic perturbationusing the second electrical sensor signal, wherein the heart soundfeature circuit is configured to identify the heart sound feature in theelectrical heart sound signal in relation to the presence of thedetected physiologic perturbation; and a trending circuit configured totrend a change in a difference between the heart sound feature in thepresence of the physiologic perturbation and in the absence of thephysiologic perturbation, and wherein the processor circuit isconfigured to declare a change in a physiologic condition of the patientaccording to the trend and provide an indication of the change to a useror process.
 2. The system of claim 1, wherein the heart sound featurecircuit is configured to identify a plurality of candidate heart soundfeatures, wherein the processor circuit includes a ranking circuitconfigured to: rank the candidate heart sound features; and select aheart sound feature for trending according to the ranking
 3. The systemof claim 2, wherein the ranking circuit is configured to rank thecandidate features according to a physiologic condition of the subject.4. The system of claim 1, wherein the second implantable sensor circuitincludes a cardiac signal sensing circuit configured to provide anelectrical representative of an intrinsic cardiac signal of the subject,and wherein the detection circuit is configured to detect a prematureventricular contraction (PVC) to trigger the heart sound featurecircuit.
 5. The system of claim 1, wherein the second implantable sensorcircuit includes an activity sensor to provide a signal representativeof a physical activity level of the subject, and wherein the detectioncircuit is configured to detect that a physical activity level of thesubject exceeds a threshold activity level from the signal to triggerthe heart sound feature circuit.
 6. The system of claim 1, wherein thesecond implantable sensor circuit includes a posture sensor, and whereinthe detection circuit is configured to detect a change in posture of thesubject to trigger the heart sound feature circuit.
 7. The system ofclaim 1, wherein the second implantable sensor circuit includes acardiac signal sensing circuit configured to provide an electricalrepresentative of an intrinsic cardiac signal of the subject, andwherein the detection circuit is configured to detect that a heart rateof the subject exceeds a threshold heart rate to trigger the heart soundfeature circuit.
 8. The system of claim 1, wherein the secondimplantable sensor circuit includes a respiration sensor to provide asignal representative of respiration of the subject, and wherein thedetection circuit is configured to detect a phase of a respiration cycleof the subject from the signal to trigger the heart sound featurecircuit.
 9. The system of claim 1, wherein the processor circuitincludes a timer circuit, and wherein the processor circuit isconfigured to initiate the identification of the heart sound feature inrelation to a time the subject receives a drug therapy.
 10. The systemof claim 1, wherein the processor circuit includes a timer circuit, andwherein the processor circuit is configured to initiate theidentification of the heart sound feature in relation to a phase of acircadian cycle of the subject.
 11. The system of claim 1, including atherapy circuit in electrical communication with the processor circuit,wherein the therapy circuit is configured to provide a therapy to thesubject, and wherein the processor circuit is configured to initiate atherapy to cause the physiologic perturbation and to initiate theidentification of the heart sound feature in a time relation to thetherapy.
 12. The system of claim 11, wherein the processor circuit isconfigured to initiate a paced cardiac contraction as the physiologicperturbation and initiate the identification of the heart sound featurein a time relation to the paced cardiac contraction.
 13. The system ofclaim 12, wherein the processor circuit is configured to initiate apremature ventricular contraction (PVC) as the physiologic perturbationand to initiate the identification of the heart sound feature in a timerelation to the PVC.
 14. A method comprising: implantably sensing anelectrical heart sound signal representative of a heart sound of asubject; identifying a feature in the heart sound signal in an absenceof a physiologic perturbation; implantably sensing a second electricalsignal related to one or more physiologic cardiovascular events in thesubject; detecting the physiologic perturbation using the second signal;identifying the feature of the heart sound signal in the presence of thedetected physiologic perturbation; trending a change in a differencebetween the heart sound feature in the presence of the physiologicperturbation and the heart sound feature in the absence of thephysiologic perturbation; identifying a change in a physiologiccondition of the subject from the trending; and providing an indicationof the change to a user or process.
 15. The method of claim 14, whereinidentifying a feature includes: identifying a plurality of candidatefeatures of the heart sound signal; ranking the candidate features; andselecting a feature for trending according to the ranking.
 16. Themethod of claim 15, wherein ranking the candidate features includesranking the candidate features according to a physiologic condition ofthe subject.
 17. The method of claim 14, wherein identifying a featureof the heart sound signal includes triggering identification of thefeature of the heart sound in relation to a detected physiologicperturbation.
 18. The method of claim 14, wherein identifying a featureof the heart sound signal includes triggering identification of thefeature of the heart sound in relation to an induced physiologicperturbation.
 19. The method of claim 14, wherein the physiologicperturbation is a premature ventricular contraction (PVC), and whereinsensing a heart sound signal includes sensing a heart sound signal inrelation to the PVC.
 20. The method of claim 14, wherein the physiologicperturbation is a change in posture of the subject, and wherein sensinga heart sound signal includes sensing a heart sound signal in relationto the change in posture.
 21. The method of claim 14, wherein thephysiologic perturbation is a heart rate of the subject exceeding athreshold heart rate, and wherein sensing a heart sound signal includessensing a heart sound signal in relation to the heart rate of thesubject exceeding the threshold heart rate.
 22. The method of claim 14,wherein the physiologic perturbation is the subject receiving drugtherapy, and wherein sensing a heart sound signal includes sensing aheart sound signal in relation to a time the subject receives the drugtherapy.
 23. The method of claim 14, wherein the physiologicperturbation is a paced cardiac contraction, and wherein sensing a heartsound signal includes sensing a heart sound signal in relation to thepaced cardiac contraction.
 24. The method of claim 14, wherein thephysiologic perturbation is a phase of a respiration cycle, and whereinsensing a heart sound signal includes sensing a heart sound signal inrelation to the phase of a respiration cycle.
 25. The method of claim14, wherein the physiologic perturbation corresponds to a circadianpattern, and wherein sensing a heart sound signal includes sensing aheart sound signal in relation to the circadian pattern.
 26. The methodof claim 14, wherein the physiologic perturbation includes activity ofthe subject exceeding a threshold activity level, and wherein sensing aheart sound signal includes sensing a heart sound signal in relation tothe activity level of the subject exceeding the threshold activitylevel.
 27. A system comprising: means for implantably sensing anelectrical heart sound signal representative of a heart sound of asubject; means for identifying a feature in the heart sound signal in anabsence of a physiologic perturbation; means for implantably sensing asecond signal related to one or more physiologic cardiovascular eventsin the subject; means for detecting the physiologic perturbation usingthe second signal; means for identifying the heart sound feature in thepresence of the detected physiologic perturbation; means for trending achange in a difference between the heart sound feature in the presenceof the physiologic perturbation and the heart sound feature in theabsence of the physiologic perturbation; means for identifying a changein a physiologic condition of the subject from the trending; and meansfor providing an indication of the change to a user or process.
 28. Asystem comprising: an implantable medical device (IMD) including: animplantable heart sound sensor circuit configured to produce anelectrical heart sound signal representative of a heart sound of asubject; a second implantable sensor circuit configured to produce asecond electrical sensor signal related to one or more physiologiccardiovascular events in the subject; a first processor circuit coupledto the heart sound sensor circuit and the second sensor circuit, whereinthe processor circuit includes: a heart sound feature circuit configuredto identify a heart sound feature in the electrical heart sound signalin an absence of a physiologic perturbation; a detection circuitconfigured to detect the physiologic perturbation using the secondsensor signal, wherein the heart sound feature circuit is configured toidentify the heart sound feature in the electrical heart sound signal inthe presence of the detected physiologic perturbation; and an externaldevice including: a second processor circuit including a trendingcircuit configured to trend a change in a difference between the heartsound feature in the presence of the physiologic perturbation and in theabsence of the physiologic perturbation, wherein the second processorcircuit is configured to declare a change in a physiologic condition ofthe subject according to the trending and provide an indication of thechange to a user or process.
 29. The system of claim 28, wherein theheart sound feature circuit is configured to identify a plurality ofcandidate heart sound features, wherein the first processor circuit isconfigured to communicate information about the heart sound signalfeatures to the external device; and wherein the second processorcircuit includes a ranking circuit configured to: rank the candidateheart sound features; and select a heart sound feature for trendingaccording to the ranking.
 30. The system of claim 29, wherein theranking circuit is configured to rank the candidate features accordingto a physiologic condition of the subject.